Circuit with extended logarithmic characteristic



July 31, 1956 c. J. LE BEL 2,757,281

CIRCUIT WITH EXTENDED LOGARITHMIC CHARACTERISTIC Filed Dec. 20. 1951 Fig. 1

/0 1/5 92%;) I- a 20 i 10 j I OUTPUT 17 '12 fzrlxirj 20 22 23 I UUTPUT L1 L-1 j INVENTOR.

CLARENCE d. A5554 zhwfmmm Unimd tates Patent O CIRCUIT 'WITH EXTENDED .LOGARIT-Hh IIC 1 CHARACTERISTIC Clarence J. Le BelfNev'v York, N. Y.

' Application December'zfl; 1951; -Serial*-No.=262',592

- Claims. (Cl. 250-27) Theinvention' relates to a circuit including an element in the circuit With a logarithmic characteristic in which the-logarithmic rangeis substantially increased. The -log2irithmic characteristic is one, in whichtheoutput' volt- ---age or current of a circuit is directly proportional to the --logarithmic of the input voltage (or current) over the l logarithmic range. -Such elements" have a resistance which varies inversely'with current-which isthe loga- -rithmic characteristic. Many crystals and other materials have this characteristic over a-certain portion of its applied voltage range; however, the germanium crystal has a logarithmic range which is; greater than that of the other known logarithmic elements. The germanium crystal has .loga'rithrhic-rangeof approximately forty-five decibels van itheinvention provides a circuit-by'which thelogarithmic range'is'substantially extended'suchas to seventyfiveiwdecibels. If desiredthe circuit may be such as'or a may be adjusted to deviate somewhat from the logarithmic i: characteristic to compensatefor the circuitor device 'conunectedlwith. the output terminals of the logarithmiccircuit.

.It:isian' object of the invention to construct a logarithmic circuit'iincluding anelement havinga logarithmic charac- :teristic-in which the range of the logarithmiccharacteristic "rdS BXwnded.

riAnother object of the invent-ion is to construct a logarithmicacircuit 'withna logarithmic rangewhich' is sub- .stantially extended by :an out of phase-voltage-being apv plied to thelcircuit, such as across an =ohmicresistor, properly'rel-ated'to the inherenb'ohmic resistance ofthe :logarithmic element.

1 Another objectnof the; invention is .to construct a simple -.logar-ithmic circuitwith an extended logarithmic range I including-the cathode bias resistor of: a'vocuum t'ube as means for introducing an out of phase voltageinto. the c1rcuit.

v A.stillfurtherobject is 'toconstruct alogarithmic circuit whichmay extendthe-rlogarithmic range'th'ereof -and/ or T .1nsert. a desired variation from the'logarithmiccharacteristic-in the range above the-normal range of theJoga- =rithmic. characteristic= and which may extend iinto 'the normal range if desired.

Other. objects of the r invention "will. be more: apparent from the following-description when taken "in connection with .theaccompanying drawings illustrating two embodiments of the invention inwhich:

. Eigure 1 shows a .d=iagrammatic logarithmic circuit .utilizing. atran-sformerto apply-an out of phase-voltage to thecircuit;

I Figure. 2 shows a diagrammatic logarithmic circuit utilizing a cathode bias resist-orof an electrontubeto-introvduce; an out-of-phase voltage into the circuit; and

. Figure 3-ShOWs-tWo rectifier-unitshaving a logarithmic characteristic connected in parallel to Iform-a logarithmic element.

Manycrystals and artificialceramic materials have a logarithmic characteristic in: that: uniform or linear :varia- .tions: in Q the voltage input will result; inlogarithmic varia- ..'.tions.in voltage output. "Asimilar.circuitmaybeused 2,757,281 Patented July 31,. 1956 for voltage-current logarithmic conversions with an extended range. Such elements are also known as varistors. With the circuit herein,the logarithmic range of a germanium crystal known as the IN 34 has been nearly doubled from 'forty-five decibels to seventy-five decibels .by -an extension ofthe range on the high voltage side thereof.

The circuit includes alogarithmic element 10 which-is diagrammatically shown as a resistance or impedance -11 having a pure*or unmodified logarithmic characteristic and connected in series "therewith an inherent resistance 12 of fixed ohmic value. Logarithmic elements have inherently this'dual logarithmic and ohmic characteristic. One or more logarithmic elements may be connected in series or parallel depending upon'the voltage or current output or input desired.

The circuit includes a'first resistor 14 having a value of resistance as' will be discussed more fully hereinafter. *For the IN 34 germanium elementthe resistance may be "roughly of'tenthousand ohms and a resistance of one hundred thousand ohms has been used satisfactorily for a single andtwo logarithmic elements. 'Means is connected in series With the logarithmic element and the input -toinsert-anout of phase voltage into theoutp'ut circuit "portion corresponding in value or related in value with the-voltageresponse of the inherent ohmic resistance characteristic'of thelogarithmic element. This'preferably is accomplished by a second-resistor 15 connected inseries h With the logarithmic element 10 and has a value ofohmic -'resistance related to-that of theinternal ohmic resistance of the logarithmic element as will appear hereinafter. Preferably to thesecond resistoris a potentiometer, so 'that'it may roughly approximate the ohmic value of'the internal ohmic resistance of the logarithmic element and 4 may be adjusted when connected to insert into the circuit at leasta-portion thereof which is of a value of'ohmic resistance properlyrelated to that inherentlyin the loga- Arithmicelement. 'The-potentiometer'for the second resistor ispreferred because thefixed ohmic resistancein- :herent in a logarithmic element is' different in each loga- 1llllII1lC element and the variable resistor enables adjustment torbe made for whatever value of ohmic resistance occurs: and also to :en'ablethe output voltage to vary from theslogarithmic characteristic either above or below-as may lbs-desired .from a consideration of the load connected with the output terminals.

An out of phase voltage is applied across the'second resistor onpotentiometer' from any suitable source't-hat shown in Figure 1 being'from a secondary winding of a transformer-whose primary-Winding 17 is connected across the circuit and preferably-across'the input terminals 19 of the circuit, a turns ratio being approximately determined by the proportion E secondary is to-E primary as the inherent resistance is to the sum'of-the inh'erent resistance and that of the first resistor. A ratio of about one thousand to one'supplies the necessary out of phase voltage. A different and higher turns ratio will be'used-if the transformer is connected across circuit elements providing a lower primary voltage. The secondary winding of the I transformer may be connected in series-with-the logarithinic element in place of the potentiometer if thedesired value of voltage can be ascertained or if the number of turns canbe varied by'taps'to adjust the sa'mej however, the potentiometer is the simpler and more practical arrangement.

The first resistor 14,- the logarithmic element 10 and the second resistor or potentiometer 15' are connected together in series and the input terminals 19 are across this series circuit. The series arrangement preferred and particularly.illustrated,yis.with the first resistor connected to thehigh voltage side of the logarithmic element and the second resistor or potentiometer connectedwi-ththe other or low voltage side; of the logarithmic-element.

v vacuum tube 24, shown as a triode.

, fixed ohmic resistance of the logarithmic element.

The second resistor may be connected between the first resistor and the logarithmic element but it is essential that the second resistor be connected with the logarithmic element in order that the output terminals 20 be connected across the logarithmic element and the means for inserting an out of phase voltage which is shown as the second resistor or potentiometer.

In the circuit of Figure 2, the second resistor or potentiometer 15 is a part of the cathode resistor 23 of a This second resistor has an approximate ohmic value properly related to the logarithmic and tube circuit as will appear and is adjustable to an ohmic value determined practically by testing the logarithmic characteristic of the output. A bypass condenser 22 is many times used with a cathode resistor and if so used, it must be connected so as not to by-pass any part of the second resistor or potentiometer.

The vacuum tube 2 includes a grid 25 having an input terminal 19:: connected therewith and the other input terminal 1% which conveniently is at the lower or low potential end of the second resistor. The vacuum tube includes a plate or anode 25 having a load resistor 27 connected therewith adapted to be connected with a source of plate potential. The first ohmic resistor 14 is connected with the plate at an input terminal 19 and through a blocking condenser 28. The first resistor, the logarithmic element, and the second resistor are connected in series as described hereinbefore. For a circuit includ ing a vacuum tube one of the input terminals 19a is the connection with the grid and the other may be at any point 1% with the cathode resistor. Output terminals 20 are provided across the logarithmic element lid and the second resistor 15.

The voltage variations across the cathode resistor, and hence the second resistor 15, is in out of phase relation with the plate voltage of the tube and therefore applies an out of phase voltage in series with the logarithmic element in the same manner as an out of phase voltage is applied across the second resistor or potentiometer utilizing a transformer of the circuit of Figure 1. The

. vacuum tube comprises means connected across or to the input terminals of the circuit and controlled by the input and inserting an out of phase voltage in series with the logarithmic element. The value of the second resistor 15 can be approximately determined by the ratio of the product of the load resistance 27 (a) and the inherent ohmic resistance (11) of the logarithmic element divided by the resistance (c) of the first resistor 14, that is, (a) (b) divided by (c). In other words the second resistor will approximately differ from the inherent ohmic resistance of the logarithmic element in the same ratio that the vacuum tube load resistance differs from the resistance of the first resistor.

The means for inserting an out of phase voltage in series with the logarithmic element in both circuits inserts a voltage corresponding with, or equal in magnitude to the voltage within the circuit occasioned by the inherent in all of the circuits illustrated the second resistance or the inserted out of phase voltage may be adjusted to vary the output voltage somewhat from a true logarithmic response either above or below the same if such variation should be desirable because of the circuit or device con nected with the output terminals 29. The term logarithmic range or extended logarithmic range as used herein means a true logarithmic response or such desired devialn the circuits described the out nium rectifiers, such as the IN 34, the logarithmic element 10 may include at least two such rectifiers 30 connected in parallel with each other and connected in reverse relation as shown in Figure 3, in order that the logarithmic operation may occur on both halves of the A. C. input wave, if A. C. input is used.

The first resistor may have a minimum resistive value roughly determined as five times the resistance of the logarithmic element at the lowest value of the current through the element. For example when extending the range to seventy-five decibels the resistance of the IN 34 logarithmic element is about two thousand ohms which means the first resistor may have a minimum resistance of ten thousand ohms. For logarithmic elements having lower resistances the first resistor may have a resistance less than ten thousand ohms, likewise, for a lower operating range for the IN 34 element where the resistance is lower, the first resistor also may be less than ten thousand ohms.

This invention provides an improvement in a circuit with extended logarithmic characteristic. It is understood that various modifications in structure, as well as changes in mode of operation, assembly, and manner of use, may and often do occur to those skilled in the art, especially after benefiting from the teachings of an invention. This disclosure illustrates the preferred means of embodying the invention in useful form.

What is claimed is:

1. A logarithmic circuit having an extended logarithmic range comprising a circuit including input terminals to which the input voltage is applied an element having a logarithmic characteristic and operated at its logarithmic characteristic over a portion of an applied voltage range and having an inherent ohmic resistive characteristic which causes deviation from the logarithmic characteristic at higher applied voltages, a first resistor having a resistance which is substantially constant with change in current and having a resistance value of at least five times the resistance of the logarithmic element at its lowest operating current value, means connected between the input terminals to generate an out-of-phase voltage with respect to the input voltage including means connected in series with the logarithmic element to which the outof-phase voltage is applied, the out-of-phase voltage corresponding to the voltage response of the inherent revoltage at least approximately corresponding to the voltage response of the inherent resistance of the logarithmic element.

3. A logarithmic circuit having an extended logarithmic range comprising a circuit having input terminals to which an input voltage is applied, an element having a logarithmic characteristic and operated at its logarithmic characteristic over a portion of an applied voltage range and having an inherent ohmic resistive characteristic which causes deviation from the logarithmic characteristic at higher applied voltages, a first resistor having a resistatnce which is substantially constant with change in current and having a resistance value of at least five times the resistance of the logarithmic element at its lowest operating current value, means connected in the circuit including therebetween at least the first resistor and generating an out-of-phase voltage with respect to the input voltage and including means connected in series with the logarithmic element to which the out-of-phase voltage is applied corresponding to the voltage response of the inherent resistive characteristic of the logarithmic element, the first resistor and the logarithmic element and the means to which the out-of-phase voltage is applied being connected between the input terminals in series, and a set of output terminals including therebetween the means to which the out-of-phase voltage is applied and the logarithmic element.

4. A logarithmic circuit having an extended logarithmic range as in claim 3 in which the means to which the out-of-phase voltage is applied is a second resistor having an ohmic value of resistance which applies an out-of-phase voltage to the circuit at least approximately corresponding to the voltage response of the inherent resistance of the logarithmic element.

5. A logarithmic circuit having an extended logarithmic range to which an input voltage is applied comprising a circuit including input terminals to which the input voltage is applied, an element having a logarithmic characteristic and operated at its lorgarithmic characteristic over a portion of an applied voltage range and having an inherent ohmic resistance, which causes deviation from the logarithmic characteristic at higher applied voltages, a first resistor having a resistance which is substantially constant with change in current and having a resistance of at least five times the resistance of the logarithmic element at its lowest operating current value a potentiometer connected with the logarithmic element and having a resistance which applies an out-of-phase voltage to the circuit at least approximately corresponding to the voltage response of the inherent resistance of the logarithmic element, the first resistor and the logarithmic element and the potentiometer being connected between the input terminals in series, a transformer having a primary winding connected across at least a portion of the circuit and a secondary winding connected across the potentiometer and applying an out-of-phase voltage with respect to the input voltages thereacross, the ratio of turns in the primary winding and secondary winding providing an out-of-phase voltage across the potentiometer at least approximating that of the inherent resistance, and output terminals including therebetween at least a portion of the potentiometer and the logarithmic element.

6. A logarithmic circuit having an extended logarithmic range as in claim 5, in which the primary winding of the transformer is connected with the input terminals.

7. A logarithmic circuit having an extended logarithmic range comprising a circuit including input terminals to which an input voltage is applied, an element having a logarithmic characteristic and operated at its logarithmic characteristic over a portion of an applied voltage range and having an inherent ohmic resistance which causes deviation from the logarithmic characteristic at higher applied voltages, at first ohmic resistor having a resistance which is substantially constant with change in current and connected in series with the logarithmic element and having a resistance of at least five times the resistance of the logarithmic element at its lowest operating current value, a vacuum tube having a plate and grid and cathode, an input terminal connected with the grid, a cathode resistor connected with the cathode and including a second ohmic resistor connected in series with the logarithmic element and the first ohmic resistor and having an ohmic value of resistance which applies an out-of-phase voltage to the circuit at least approximately corresponding to the voltage response of the inherent resistance of the logarithmic element, a load resistor connected with the plate, the tube applying an out-of-phase voltage with respect to the input voltage at the input terminals across the second resistor corresponding in value to the voltage across the equivalent ohmic resistive characteristic of the logarithmic element, the first resistor being connected with the plate, and output terminals including therebetween the second resistor and the logarithmic element.

8. A logarithmic circuit as in claim 7 in which the second resistor is a potentiometer having on ohmic value approximating the product of the plate load resistance and the inherent resistance of the logarithmic value divided by the resistance of the first resistor.

9. A logarithmic circuit having an extended logarithmic range comprising a circuit having input terminals to which an input voltage is applied, an element having a logarithmic characteristic and operated at its logarithmic characteristic over a portion of an applied voltage range and having an inherent ohmic resistive characteristic which causes deviation from the logarithmic characteristic at higher applied voltages, a first resistor having a resistance which is substantially constant with change in current and having a resistance value of at least five times the resistance of the logarithmic element at its lowest operating current value, a transformer having a primary winding and a secondary winding the latter being connected in series with the logarithmic element, the primary winding being connected across at least the first resistor; the first ohmic resistor and the logarithmic element and the secondary winding being connected between the input terminals in series, the primary winding and secondary winding having a turns ratio to provide across the secondary winding an out-of-phase voltage with respect to the input voltage at least approximately that of the inherent ohmic resistance of the logarithmic element, and output terminals including therebetween the secondary winding and logarithmic element.

10. A logarithmic circuit having an extended logarithmic range comprising a circuit having input terminals to which an input voltage is applied, a first resistor having a resistance which is substantially constant with change in current and having a resistance value of at least five times the resistance of the logarithmic element at its lowest operating current value, an element having a logarithmic characteristic and operated at its logarithmic characteristic over a portion of an applied voltage range and having an inherent ohmic resistive characteristic which causes deviation from the logarithmic characteristic at higher applied voltages and connected in series with the first resistor, a transformer having a primary winding and a secondary winding the latter being connected in series with the logarithmic element, the primary winding being connected between the input terminals, the first ohmic resistor and the logarithmic element and the secondary winding being connected between the input terminals, the primary winding and secondary winding providing across the secondary winding an out-ofphase voltage with respect to the input voltage and the voltage ratio between windings being approximately the ratio of the inherent ohmic resistance of the logarithmic element to the sum of the first resistor and the inherent ohmic resistance, and output terminals including therebetween the secondary winding of the transformer and the logarithmic element.

References Cited in the file of this patent UNITED STATES PATENTS 1,756,816 Dolmage Apr. 29, 1930 1,969,657 McCaa Aug. 7, 1934 2,103,490 Peterson Dec. 28, 1937 2,367,625 Short Jan. 16, 1945 2,390,773 Brinton Dec. 11, 1945 2,401,404 Bedford June 4, 1946 2,428,541 Bagley Oct. 7, 1947 2,501,263 Cherry Mar. 21, 1950 

